Multilayer transparent, biaxially oriented polyester film

ABSTRACT

The invention relates to a transparent, biaxially oriented polyester film composed of at least one base layer (B) which comprises at least 80% by weight of thermoplastic polyester, and of, applied to this base layer (B), a transparent, high-gloss outer layer (A). The outer layer (A) of the invention comprises a specific pigment system which provides improved winding of the film and leads to a highly transparent film with a high-gloss surface (A). The invention further relates to a process for producing the film. The film may be used as a packaging film, in particular for metalizing or for ceramic coating, or for applications in the industrial sector, e.g. as a substrate for stamping films, and in particular to its use on high-speed packaging machinery.

BACKGROUND OF THE INVENTION

[0001] The invention relates to a transparent, biaxially orientedpolyester film made from at least one base layer (B) which comprises atleast 80% by weight of thermoplastic polyester, and from, applied tothis base layer (B), at least one transparent outer layer (A). Theinvention further relates to a process for producing the film and to theuse of the film.

[0002] The outer layer (A) comprises a specific pigment system whichprovides improved winding of the film and leads to a highly transparentfilm with a high-gloss surface (A). The film has very high suitabilityfor use as packaging film (in particular for metalizing or for ceramiccoating) and for applications in the industrial sector (e.g. as asubstrate for stamping films).

[0003] In the packaging industry there is a high level of demand forhighly transparent and high-gloss plastic films, e.g. biaxially orientedpolypropylene films or biaxially oriented polyester films. These highlytransparent, high-gloss films have a characteristic appearance and givethe packaging, for example, an appearance which is particularlyattractive and therefore particularly effective for promotionalpurposes. The problem with films subject to high requirements for theoptical properties of the film is, inter alia, their windability, whichdoes not always meet the requirements set, in particular if the machinerolls are processed to give wide customer rolls of high running length.

[0004] The prior art shows how the optical properties, in particular theglass and the haze, of biaxially oriented polyester films can beimproved.

[0005] EP-A-0 347 646 describes a biaxially oriented polyester filmwhich has at least one outer layer (A) which comprises a concentrationof from 0.5 to 50% of a filler, the diameter of this filler being in aparticular ratio to the thickness of the outer layer. The outer layeralso has a certain thickness and a certain degree of crystallization,which is determined with the aid of Raman spectroscopy. The topographyof the outer layer A makes the film particularly suitable for magneticrecording tapes. The specification gives no information concerning thetransparency of the film or the gloss achieved on the outer layer A. Afilm produced as in EP 347 646 did not have the desired transparency,nor the desired glossy surface. The gloss of this surface and the hazeof the film are outside the range claimed in this specification.

[0006] EP-A-0 514 129 describes a transparent multilayer film whichencompasses a primary layer substrate made from polymer material whichhas, at least on one of its surfaces, a secondary layer made frompolymer material and having glass beads and silicon dioxide particles atparticular concentrations and in particular size distribution. Thearrangement of the secondary layer may be on one side or on both sidesof the primary layer substrate. The film improves haze and processingproperties, but the specification teaches nothing concerning improvementof the gloss and of the windability of the film.

[0007] EP-A-0 604 057 describes a transparent multilayer film whichencompasses a primary layer substrate made from polymer material andsubstantially free from fillers, and having, at least on one of itssurfaces, a secondary layer which is made from polymer material andwhich comprises a concentration of from 100 to 1000 ppm of siliconeresin with a median particle diameter of from 1.5 to 12.5 μm. Adisadvantage of the silicone particles is that these are comparativelyexpensive and do not represent an acceptable solution for the packagingmarket. In addition, films equipped with pigments of this type tend totelescope on winding. Nor does that specification give any indication atall as to how the topography of such a film is to be adjusted forsimultaneous improvement of gloss and windability.

[0008] DE-A-16 94 404 describes a laminate with a plurality of layers ofan oriented crystalizable thermoplastic film, in which at least one ofthe outer layers comprises an additive. The additives are conventionalinert inorganic or organic particles which, in the case of the inertparticles such as SiO₂, are added at a concentration of from 1 to 25% byweight to the outer layers. The particle size here is from 2 to 20 μm.The laminates may be metalized with aluminum for decorative purposes orused for magnetic tapes. Although the teaching of this specificationenables the processing properties of the film to be improved, thespecification gives no teaching concerning improvements of gloss and ofthe windability of the film.

[0009] EP-B-0 061 769 describes a magnetic recording medium composed ofa biaxially oriented polyester film and of a thin magnetic metalliclayer on the surface A of the polyester film. Where appropriate, thereis also a lubricant layer present on the other surface B of thepolyester film. The film is characterized in that the coated surface A

[0010] a) has an average roughness R_(a) (peak to valley value) of notmore than 5 nm (60 nm),

[0011] b) the number of the protrusions with a height of from 0.27 to0.54 μm is from 0 to 0.2/mm², and

[0012] c) is free from protrusions with a height greater than 0.54 μm.

[0013] A disadvantage of the film is that the surface A tends to block,and the film therefore lacks good processability. The specificationgives no teaching concerning the gloss of the film or its haze orwindability.

[0014] EP-B-0 088 635 describes a coextruded biaxially orientedpolyester film with at least two layers, of which one layer A iscomposed of thermoplastic resin and one layer B comprises thermoplasticresin and fine particles. The film is characterized in that the surfaceroughness R_(a) of the outer surface of the layer A is less than 5 nm,and in that the outer surface of the layer B either

[0015] i) is a surface with surface roughness R_(a) of from 5 to 40 nmand has a large number of depressions and a large number of protrusions,these having a particular arrangement, or

[0016] ii) is a surface which has protrusions formed on a flat plane,its surface being covered by a layer C which is composed of a lubricantand which has a surface roughness R_(a) of from 5 to 40 nm.

[0017] A disadvantage of the film surface A is that it blocks againstitself and against certain other surfaces (e.g. rubber rolls). The filmcannot be processed cost-effectively, and in particular duringvacuum-metalizing the high blocking tendency of the film gives it atendency toward break-offs, and this can be associated with majorcost-effectiveness problems. The film is not suitable for the objectset. In addition, the film has unsatisfactory haze.

[0018] EP-B-0 502 745 describes a coextruded biaxially orientedpolyester film with at least three layers, of which an outer layer A

[0019] a) comprises inorganic particles with a mean primary particlesize D in the range from 1 to 100 nm and complying with the equationD<T<200·D, where T is the thickness of layer A,

[0020] b) comprises particles B with a mean primary particle size Dl inthe range from 0.3 to 2 μm and with a primary particle size distributionhaving a relative standard deviation of not more than 0.6, and

[0021] c) where the mean primary particle size D of the particles A issmaller than the mean primary particle size Dl of the particles B.

[0022] The teaching of this specification in particular improves theprocessing performance of the film. The specification gives no teachingconcerning improvement of the gloss of the film, or of its haze orwindability.

[0023] It was then an object of the present invention to provide acoextruded, biaxially oriented polyester film which has very goodoptical properties and in particular has very high gloss and very lowhaze. The film should, furthermore, be very easy to wind, in particularwhen the machine roll is processed to give wide customer rolls of highrunning length. In addition, the film is intended to be very easy toproduce and to process and to provide a good oxygen barrier aftermetalizing or after coating with oxidic materials. In summary, theobject was to provide a film with the following combination of features:

[0024] high gloss

[0025] low haze

[0026] good winding

[0027] low oxygen transmission of the film after metalizing or aftercoating with oxidic materials

[0028] low coefficients of friction.

[0029] The gloss of the film is intended to be greater than 170, and thehaze lower than 2.5%. The film is intended to be very easy to wind, andthis means in particular that there are to all intents and purposes nolongitudinal corrugations in customer rolls produced from a machineroll. This applies in particular when these are wide customer rolls ofhigh running length. It is intended that less than 1.0 cm³ of oxygen persquare meter and per day will diffuse through the metalized film when itis subjected to air at a pressure of 1 bar. In respect of otherproperties, the quality of the film is intended to be at least equal tothat of the known packaging films of this type. The film is alsointended to be simple and cost-effective to produce, and to be very easyto process on conventional machinery. The coefficient of friction onboth surfaces is intended to be less than 0.6.

[0030] A further intention is to ensure that cut material arising duringfilm production can be reintroduced to the manufacturing process asregrind in amounts of up to 60% by weight, based on the total weight ofthe film, without any resultant significant adverse effect on thephysical or optical properties of the film.

BRIEF DESCRIPTION OF THE INVENTION

[0031] According to the invention, the object is achieved by providing atransparent, biaxially oriented polyester film made from at least onebase layer (B) which comprises at least 80% by weight of thermoplasticpolyester, and from at least one transparent, high-gloss outer layer(A), where the transparent outer layer (A) comprises an amount in therange from 0.05 to 0.5% by weight, based on the total weight of theouter layer (A), of a pigment system which has the following features

[0032] a) the median grain diameter (d₅₀) is in the range from 1.5 to 5μm and

[0033] b) the spread of the distribution of the grain size, expressed bythe SPAN 98, is less than or equal to 1.9.

[0034] The subclaims give preferred embodiments of the invention, andthese are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 shows a customer role with clearly visible corrugations.

[0036]FIG. 2 shows a graph for the determination of d₅₀ of particles.

[0037]FIG. 3 shows a graph for the determination of d₁₀ and d₉₈ofparticles.

DETAILED DESCRIPTION OF THE INVENTION

[0038] According to the invention, the film has at least two layers andthen encompasses the base layer (B) and the glossy outer layer (A).

[0039] The base layer (B) preferably comprises at least 90% by weight ofa thermoplastic polyester. Polyesters suitable for this purpose arethose made from ethylene glycol and terephthalic acid (polyethyleneterephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid[poly(-1,4-cyclohexanedimethylene terephthalate) PCDT], or else madefrom ethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters ofwhich at least 90 mol %, preferably at least 95 mol %, is composed ofethylene glycol units and terephthalic acid units, or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. In one particularlypreferred embodiment, the base layer is composed of polyethyleneterephthalate homopolymer. The remaining monomer units derive from otheraliphatic, cycloaliphatic or aromatic diols and, respectively,dicarboxylic acids.

[0040] Other examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branchedaliphatic glycols having up to 6 carbon atoms. Among the cycloal iphaticdiols, mention should be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of other suitable aromatic diols have theformula HO—C₆H₄—X—C₆H₄—OH, where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—,—S— or —SO₂—. Bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also verysuitable. Other aromatic dicarboxylic acids are preferablybenzenedicarboxylic acids, naphthalene dicarboxylic acids (such asnaphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylicacids (in particular biphenyl-4,4′-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylicacids. Among the cycloaliphatic dicarboxylic acids, mention should bemade of cyclohexanedicarboxylic acids (in particularcyclohexane-1,4-dicarboxylic acid). Among the aliphatic dicarboxylicacids, the C₃-C₁₉ alkanediacids are particularly suitable, and thealkane moiety here may be straight-chain or branched.

[0041] One way of preparing the polyesters is the transesterificationprocess. Here, the starting materials are dicarboxylic esters and diols,which are reacted using the customary transesterification catalysts,such as the salts of zinc, of calcium, of lithium, of magnesium or ofmanganese. The intermediates are then polycondensed in the presence ofwell-known polycondensation catalysts, such as antimony trioxide ortitanium salts. Another equally good preparation method is the directesterification process in the presence of polycondensation catalysts.This starts directly from the dicarboxylic acids and the diols. For theouter layer (A) or other outer layers use may in principle be made ofpolymers which are the same as those used for the base layer (B).Besides these, there may also be other materials present in the outerlayers, and in this case the outer layers are preferably composed of amixture of polymers or of a copolymer or of a homopolymer whichcomprises ethylene isophthalate units and/or ethylene 2,6-naphthalateunits and/or ethylene terephthalate units. Up to 10 mol % of thepolymers may be composed of other comonomers.

[0042] To achieve the desired high gloss, the outer layer (A) comprisesat least the pigment system of the invention, in an effective amount inthe range from 0.05 to 0.5% by weight, based on the weight of the outerlayer (A). In the preferred embodiment of the film of the presentinvention, the pigment concentration is in the range from 0.055 to 0.45%by weight, particularly preferably from 0.06 to 0.4% by weight.

[0043] Typical particle systems are inorganic and/or organic particles,such as calcium carbonate, amorphous silica, talc, magnesium carbonate,barium carbonate, calcium sulfate, barium sulfate, lithium phosphate,calcium phosphate, magnesium phosphate, aluminum oxide, lithiumfluoride, the calcium, barium, zinc or manganese salts of thedicarboxylic acids used, carbon black, titanium dioxide, kaolin orcrosslinked polymer particles, e.g. polystyrene particles or acrylateparticles.

[0044] It is also possible to select mixtures of two or more differentpigment systems, or mixtures of pigment systems of the same chemicalmakeup but of different particle size. The particles may be added to thepolymers of each layer of the film in the respective advantageousconcentrations, e.g. as a glycolic dispersion during thepolycondensation, or by way of masterbatches during extrusion.

[0045] To achieve the desired high gloss and the high transparency,there is a comparatively low level of filling of the outer layer (A)with inert particles. The concentration of the inert particles in theouter layer (A) in the preferred embodiment is from 0.055 to 0.45% byweight, and in the particularly preferred embodiment it is from 0.06 to0.4% by weight, and is substantially dependent on the optical propertiesto be achieved by the film, and on its running properties.

[0046] If the concentration of the particles is greater than thatprescribed, the desired optical properties (high transparency and highgloss) are no longer achieved. The film becomes cloudy and the surfacebecomes dull (matt). If, on the other hand, the concentration of theparticles is smaller than that prescribed, this adversely affects therunning properties and, respectively, the processing properties of thefilm. The surface of the film tends to block, making it impossible toensure good processability, in particular on high-speed machinery.

[0047] Preferred particles are SiO₂ in colloidal or in chain-type form.These particles are bound very effectively into the polymer matrix.

[0048] It has been found that the grain diameter and the scattering(spread) of the diameter of the pigment system used affect the gloss,the haze, and in particular the windability of the film. It has beenfound that a smaller spread of the median diameter brings about a moreuniform thickness of air layer between the laps of film and thus reducesany tendency toward blocking of the laps of film against one another,or, respectively, improves windability.

[0049] To achieve the lowest possible haze, the highest possible gloss,and good windability, the outer layer (A) of the film of the presentinvention comprises a pigment system in which the median diameter (thed₅₀) is in the range from 1.5 to 5.0 μm, and the spread (expressed viathe SPAN 98) is less than 1.9.

[0050] In the preferred embodiment, the outer layer (A) of the film ofthe present invention comprises a pigment system in which the mediandiameter is in the range from 1.6 to 4.9 μm and the spread is less than1.8. In the particularly preferred embodiment, the outer layer (A) ofthe film of the present invention comprises a pigment system in whichthe median diameter is in the range from 1.7 to 4.8 μm and the spread isless than 1.7.

[0051] If, in contrast, the outer layer (A) of the film comprises apigment system in which the median diameter and the spread are outsidethe inventive range, this has a particularly adverse effect on thewindability of the film. There can also be an adverse resultant affecton the transparency of the film and its gloss.

[0052] If the outer layer (A) of the film comprises a pigment system inwhich the median diameter is greater than 5.0 μm and the spread isgreater than 1.9, there is in particular an adverse effect on thewindability of the film. In these films there is an exacerbated tendencytoward blocking of the film laps against one another. The customer rollsproduced from the machine roll have an increased tendency towardformation of longitudinal corrugations, as illustrated in FIG. 1, whereone or more corrugation(s) become(s) clearly visible around theperiphery of the roll, this/these being more easily indented than thesurroundings. These corrugations markedly restrict the process abilityand the use of the film. The longitudinal corrugations impressed intothe films (in some cases irreversibly) are namely visible in the productduring/after further processing (e.g. metalization, printing, stampedfilm application), and are an optical/mechanical defect impairing theusability of the film. Wide rolls (above 1.5 m) and rolls of highrunning length (greater than 24 000 m) in particular have a strongtendency toward formation of these longitudinal corrugations. Inaddition, it has been found that films with the abovementioned pigmentsystem which is unfavorable for the purposes of the present inventionhave an exacerbated tendency toward loss of transparency and towardreduced gloss.

[0053] If, in contrast, the outer layer (A) of the film comprises apigment system in which the median diameter is smaller than 1.5 μm andthe spread is greater than 1.9, this likewise can become noticeable inimpaired winding and impaired optical properties. The large number ofsmall pigment particles (broad distribution of pigment system and smallparticle diameter) form a large number of scattering centers in thefilm, and these reduce not only the transparency of the film but alsoits gloss. Another result here is impairment of the windability of theroll, and indeed to the degree described above. These films also have avery pronounced tendency toward telescoping or off-center running of therolls.

[0054] In one useful embodiment, the gloss-appearance outer layer (A) isdescribed by the following further parameters:

[0055] a) The roughness of the film, expressed by its R_(a)-value, is inthe range from 30 to 150 nm, preferably from 35 to 140 nm, particularlypreferably from 40 to 130 nm.

[0056] Roughness values smaller than 30 nm have an adverse effect onrunning properties when the claimed pigment system is used, and valuesgreater than 130 nm impair the optical properties of the film.

[0057] b) The value measured for surface gas flow is in the range from 4to 200 s, preferably in the range from 5 to 180 s. At values above 200the windability of the film is adversely affected.

[0058] The base layer (B) may also comprise customary additives, such asstabilisers and/or pigments (fillers). Phosphorous compounds, such asphosphoric acid or phosphoric esters, are advantageously used asstabilizers.

[0059] Typical pigments (fillers) for the base layer (B) are, as statedfor the outer layer(s), inorganic and/or organic particles, such ascalcium carbonate, amorphous silica, talc, magnesium carbonate, bariumcarbonate, calcium sulfate, barium sulfate, lithium phosphate, calciumphosphate, magnesium phosphate, aluminum oxide, LiF, the cadmium,barium, zinc, or manganese salts of the dicarboxylic acids used, carbonblack, titanium dioxide, kaolin, or crosslinked polystyrene particles orcrosslinked acrylate particles.

[0060] In one particularly advantageous embodiment, the base layer (B)comprises only the pigments introduced by way of the regrind. The filmthus produced has particularly low haze.

[0061] In one advantageous embodiment, the film is composed of threelayers, the base layer (B) and outer layers (A) and (C) applied to thetwo sides of this base layer, the outer layer (A) being high-gloss andcomprising the inventive amount of pigment system. In one particularlyadvantageous embodiment, the three-layer film has a symmetricalstructure and is composed of the base layer (B) and of, applied to thetwo sides of this base layer (B), outer layers (A) and (C), the outerlayers (A) and (C) being high-gloss and comprising the inventive pigmentsystem.

[0062] Between the base layer (B) and the outer layers there may, whereappropriate, also be an intermediate layer. This may again be composedof the polymers described for the base layer (B). In one particularlypreferred embodiment, the intermediate layer is composed of thepolyester used for the base layer (B). The intermediate layer may alsocomprise the customary additives described. The thickness of theintermediate layer is generally greater than 0.3 μm, preferably in therange from 0.5 to 15 μm, in particular in the range from 1.0 to 10 μm,particularly preferably in the range from 1.0 to 5 μm.

[0063] In the particularly advantageous three-layered embodiment of thefilm of the invention, the thickness of the outer layers (A) and (C) isgenerally in the range from 0.1 to 5.0 μm, preferably in the range from0.2 to 4.5 μm, and particularly preferably in the range from 0.3 to 4.0μm, where the outer layers (A) and (C) may have identical or differentthickness.

[0064] The overall thickness of the polyester film of the invention mayvary within certain limits. It is from 3 to 80 μm, in particular from 4to 50 μm, preferably from 5 to 30 μm, the base layer (B) making up aproportion which is preferably from 5 to 97% of the overall thickness.

[0065] The polymers for the base layer (B) and the two outer layers (A)and (C) are fed to three extruders for film production. Any foreignbodies or contamination present may be removed from the polymer meltprior to extrusion by way of suitable filters. The melts are thenextruded through a coextrusion die to give flat melt films, andlaminated. The multilayer film is then drawn off and solidified with theaid of a chill roll and, where appropriate, other rolls.

[0066] The invention also provides a process for producing the polyesterfilm of the invention by the coextrusion process known from theliterature.

[0067] The procedure for this process is that the melts corresponding tothe separate layers (A), (B), and, where appropriate, (C) of the filmare coextruded through a flat-film die, the resultant film is drawn offon one or more rolls for solidification, and the film is then biaxiallystretched (oriented), and the biaxially stretched film is heat-set and,where appropriate, corona- or flame-treated on that surface intended fortreatment, and is then wound up.

[0068] The biaxial stretching (orientation) is generally carried outsequentially, the type of sequential biaxial stretching preferred beingthat which begins with longitudinal stretching (in the machinedirection) and then transverse stretching (perpendicularly to themachine direction).

[0069] As is usual in coextrusion, the polymer or the polymer mixturesfor the separate layers is/are first compressed and plasticized in anextruder, at which juncture any additives used may already be present inthe polymer or the polymer mixture. The melts are then simultaneouslyextruded through a flat-film die (slot die), and the extruded multilayermelt is drawn off on one or more take-off rolls, whereupon the meltcools and solidifies to form a prefilm.

[0070] The biaxial stretching is generally carried out sequentially,preferably stretching the prefilm first longitudinally (i.e. in themachine direction=MD) and then transversely (i.e. perpendicularly to themachine direction=TD). This leads to orientation of the polymer chainsin space. The longitudinal stretching may be carried out with the aid oftwo rolls rotating at different speeds corresponding to the desiredstretching ratio. For the transverse stretching, use is generally madeof an appropriate tenter frame, clamping both edges of the film and thendrawing toward the two sides at an elevated temperature.

[0071] The temperature at which the stretching is carried out may varywithin a relatively wide range, and depends on the properties desiredfrom the film. The longitudinal stretching is generally carried out at atemperature in the range from 80 to 130° C., and the transversestretching in the range from 90 to 150° C. The longitudinal stretchingratio is generally in the range from 2.5:1 to 6:1, preferably from 3:1to 5.5:1. The transverse stretching ratio is generally in the range from3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1. After the longitudinalstretching and prior to the transverse stretching, one or both surfacesof the film may be in-line coated by known processes. The in-linecoating may serve, for example, to give improved adhesion of a metallayer or of any printing ink subsequently to be applied, or else toimprove antistatic performance or processing performance.

[0072] For producing a film with a good oxygen barrier (afterappropriate metalization or after coating with ceramic substances), andwith improved winding and improved transparency, it has provenadvantageous for the planar orientation Δp of the film to be greaterthan 0.165, preferably greater than 0.166, and very preferably greaterthan 0.167.

[0073] The significant variables affecting the planar orientation Δphave been found to be the process parameters for longitudinal andtransverse stretching, and also the SV of the raw material used. Theprocess parameters include in particular the longitudinal and transversestretching ratios (λ_(MD) and λ_(TD)), the longitudinal and transversestretching temperatures (T_(MD) and T_(TD)), the film web speed, and thenature of the stretching, in particular that in the longitudinaldirection of the machine. For example, if the planar orientation Δpobtained on a machine is 0.163 using the parameter set λ_(MD)=3.7 andλ_(TD)=3.8, the stretching temperatures longitudinally and transverselybeing T_(MD)=80-123° C. and T_(TD)=80-126° C., then lowering thelongitudinal stretching temperature to T_(MD)=80-118° C. or lowering thetransverse stretching temperature to T_(TD)=80-122° C. or increasing thelongitudinal stretching ratio to λ_(MD)=4.5 or increasing the transversestretching ratio to λ_(TD)=4.0 gives a planar orientation Δp within thedesired range. The film web speed here was 370 m/min and the SV of thematerial was about 730. The data given for longitudinal stretching arebased on what is known as LTEP stretching, composed of alow-level-orienting stretching step (LOE=Low Orientation Elongation) andof a highly orienting stretching step (REP=Rapid Elongation Process).The ratios obtained using other stretching equipment are in principleidentical, but there may be very slight differences in the numericalvalues for the respective process parameters. The temperatures statedare based on the respective roll temperatures for longitudinalstretching and on the film temperatures measured by IR for transversestretching.

[0074] In the heat-setting which follows, the film is held at atemperature of from 150 to 250° C. for a period of from about 0.1 to 10s. The film is then wound up in the usual way.

[0075] After the biaxial stretching, it is preferable for one or bothsurfaces of the film to be corona- or flame-treated by one of the knownmethods. The intensity of treatment selected is generally such as togive the film a surface tension in the range above 45 mN/m.

[0076] The film may also be coated to achieve other desired properties.Typical coatings are layers with adhesion-promoting, antistatic,slip-enhancing, or release effect. It is, of course, possible for theseadditional layers to be applied to the film by in-line coating usingaqueous dispersions prior to the transverse stretching step.

[0077] The film of the invention has excellent optical properties, i.e.high gloss and high transparency, very good winding performance, andvery good processing performance.

[0078] In addition, it has been ensured that the cut material (regrind)arising during film production can be reintroduced to the extrusionprocess in amounts in the range from 20 to 60% by weight, based on thetotal weight of the film, without any significant resultant adverseeffect on the physical properties of the film, in particular itsappearance.

[0079] The film therefore has quite excellent suitability for use inflexible packaging, and specifically wherever its excellentprocessability can be utilized to full advantage, in particular onhigh-speed packaging machinery.

[0080] The table below (Table 1) gives the most important filmproperties of the invention again at a glance. TABLE 1 ParticularlyInventive range Preferred preferred Unit Test method Gloss,20° >170 >175 >180 DIN 67530 COF (Friction) <0.6 <0.55 <0.50 DIN 53375Average roughness R_(a) 30-150 35-140 40-130 nm DIN 4768. Cut-off of0.25 mm Range of values 4-200 5-180 6-160 sec internal measured forsurface gas flow Other film properties Haze <2.5 <2.3 <2.1 % ASTM-D1003-52 Planar orientation >0.165 >0.166 ≧0.167 internal Δp (optional)Oxygen transmission <1.0 <0.95 <0.9 cm³/(m².d.bar) DIN 53380 aftermetalization of layer A

[0081] The following test methods were utilized for the purposes of thepresent invention to characterize the raw materials and the films:

[0082] SV (standard viscosity)

[0083] Standard viscosity SV (DCA) is measured in dichloroacetic acid bya method based on DIN 53726.

[0084] Intrinsic viscosity (IV) is calculated as follows from standardviscosity

IV(DCA)=6.907·10⁻⁴ SV(DCA)+0.063096

[0085] Coefficient of Friction (COF)

[0086] Coefficient of friction was determined to DIN 53 375. Thecoefficient of sliding friction was measured 14 days after production.

[0087] Surface Tension

[0088] Surface tension was determined by what is known as the ink method(DIN 53 364).

[0089] Haze

[0090] Hölz haze was determined by a method based on ASTM-D 1003-52 but,in order to utilize the most effective measurement range, measurementswere made on four laps of film laid one on top of the other, and a 1°slit diaphragm was used instead of a 4° pinhole.

[0091] Gloss

[0092] Gloss was determined to DIN 67 530. Reflectance was measured,this being an optical value characteristic of a film surface. Based onthe standards ASTM-D 523-78 and ISO 2813, the angle of incidence was setat 20° or 60°. A beam of light hits the flat test surface at the setangle of incidence and is reflected and/or scattered thereby. Aproportional electrical variable is displayed representing light rayshitting the photoelectronic detector. The value measured isdimensionless and must be stated together with the angle of incidence.

[0093] Surface Gas Flow Time

[0094] The principle of the test method is based on the air flow betweenone side of the film and a smooth silicon wafer sheet. The air flowsfrom the surroundings into an evacuated space, and the interface betweenfilm and silicon wafer sheet acts to resist the flow.

[0095] A round specimen of film is placed on a silicon wafer sheet inthe middle of which there is a hole providing the connection to thereceiver. The receiver is evacuated to a pressure below 0.1 mbar. Thetime in seconds taken by the air to establish a pressure rise of 56 mbarin the receiver is determined. Test conditions: Test area 45.1 cm²Weight applied 1276 g Air temperature 23° C. Humidity 50% relativehumidity Aggregated gas volume 1.2 cm³ Pressure difference 56 mbar

[0096] Determination of Planar Orientation Δp

[0097] Planar orientation is determined by measuring the refractiveindex with an Abbe refractometer according to internal operatingprescription 24. Preparation of specimens: Specimen size and length:from 60 to 100 mm Specimen width: corresponds to prism width of 10 mm

[0098] To determine n_(MD) and n_(a)(=n_(z)), the specimen to be testedhas to be cut out from the film with the running edge of the specimenrunning precisely in the direction TD. To determine n_(TD) andn_(a)(=n_(z)), the specimen to be tested has to be cut out from the filmwith the running edge of the specimen running precisely in the directionMD. The specimens are to be taken from the middle of the film web. Caremust be taken that the temperature of the Abbe refractometer is 23° C.Using a glass rod, a little diiodomethane (N=1.745) ordiiodomethane-bromonaphthalene mixture is applied to the lower prism,which has been cleaned thoroughly before the test. The refractive indexof the mixture must be greater than 1.685. The specimen cut out in thedirection TD is firstly laid on top of this, in such a way that theentire surface of the prism is covered. Using a paper wipe the film isnow firmly pressed flat onto the prism, so that it is firmly andsmoothly positioned thereon. The excess liquid must be sucked away. Alittle of the test liquid is then dropped onto the film. The secondprism is swung down and into place and pressed firmly into contact. Theright-hand knurled screw is then used to turn the indicator scale untila transition from light to dark can be seen in the field of view in therange from 1.62 to 1.70. If the transition from light to dark is notsharp, the colors are brought together using the upper knurled screw insuch a way that only one light and one dark zone are visible. The sharptransition line is brought to the crossing point of the two diagonallines (in the eyepiece) using the lower knurled screw. The value nowindicated on the measurement scale is read off and entered into the testrecord. This is the refractive index n_(MD) in the machine direction.The scale is now turned using the lower knurled screw until the rangevisible in the eyepiece is from 1.49 to 1.50.

[0099] The refractive index n_(a) or n_(z) (in the direction of thethickness of the film) is then determined. To improve the visibility ofthe transition, which is only weakly visible, a polarization film isplaced over the eyepiece. This is turned until the transition is clearlyvisible. The same considerations apply as in the determination ofn_(MD). If the transition from light to dark is not sharp (colored), thecolors are brought together using the upper knurled screw in such a waythat a sharp transition can be seen. This sharp transition line isbrought into the crossing point of the two diagonal lines using thelower knurled screw, and the value indicated on the scale is read offand entered into the table.

[0100] The specimen is then turned, and the corresponding refractiveindices n_(MD) and n_(a) (=n_(z)) of the other side are measured andentered into an appropriate table.

[0101] After determining the refractive indices in, respectively, thedirection MD and the direction of the thickness of the film, thespecimen strip cut out in the direction MD is placed in position and therefractive indices n_(TD) and n_(a) (=n_(z)) are determined accordingly.The strip is turned over, and the values for the B side are measured.The values for the A side and the B side are combined to give averagerefractive indices. The orientation values are then calculated from therefractive indices using the following formulae:

Δn=n _(MD) −n _(TD)

Δp=(n _(MD) +n _(TD))/2−n _(z)

n _(av)=(n _(MD) +n _(TD) +n _(z))/3

[0102] Measurement of Median Grain Diameter d₅₀

[0103] The median grain diameter d₅₀ was determined by laser on aMalvern MasterSizer by the standard method (examples of othermeasurement devices are the Horiba LA 500 or Sympathec Helos, which usethe same principle of measurement). For the tests, the specimens wereplaced in a cell with water, and this was then placed into the testdevice. The test procedure is automatic and includes the mathematicaldetermination of the d₅₀ value.

[0104] The d₅₀ value here is determined as defined from the (relative)cumulative particle size distribution curve: the point of intersectionof the 50% ordinate with the cumulative curve directly gives the desiredd₅₀ value on the abscissa axis, as shown more precisely in FIG. 2.

[0105] Measurement of SPAN 98

[0106] The test device used to determine SPAN 98 was the same as thatdescribed above for the determination of median diameter d₅₀. SPAN 98 isdefined here as follows: ${SPAN98} = \frac{d_{98} - d_{10}}{d_{50}}$

[0107] The (relative) cumulative particle size distribution curve isagain used as a basis for determining d₉₈ and d₁₀. The point ofintersection of the 98% ordinate value with the cumulative curve givesthe desired d₉₈ value directly on the abscissa axis, and the point ofintersection of the 10% ordinate value with the cumulative curve givesthe desired d₁₀ value directly on the abscissa axis, as shown moreprecisely in FIG. 3.

EXAMPLE 1

[0108] Chips made from polyethylene terephthalate (prepared by thetransesterification process with Mn as transesterification catalyst, Mnconcentration: 100 ppm) were dried at 150° C. to residual moisture below100 ppm and fed to the extruder for the base layer (B).

[0109] Alongside this, chips were prepared from a polyethyleneterephthalate (prepared by the transesterification process with Mn astransesterification catalyst, Mn concentration: 100 ppm) and dried at atemperature of 150° C. to residual moisture below 100 ppm, and fed,together with the filler of the invention, to the extruders for theouter layers (A) and (C).

[0110] A transparent, three-layer film of ABC structure and with a totalthickness of 12 μm was then produced by coextrusion followed by stepwiselongitudinal and transverse orientation. The thickness of each outerlayer is given in Table 2. Outer layer (A) was a mixture made from: 90%by weight of polyethylene terephtha late with SV of 800 and 10% byweight of masterbatch made from 99% by weight of polyethyleneterephthalate and 1.0% by weight of silica particles ( ®Sylysia 430 fromFuji, Japan) with d₅₀ value of 2.5 μm and SPAN 98 of 1.4. Base layer(B): 100.0% by weight of polyethylene terephthalate with SV of 800 Outerlayer (C) was a mixture made from: 90.0% by weight of polyethyleneterephthalate with SV of 800 and 10.0% by weight of masterbatch madefrom 99% by weight of polyethylene terephthalate, 1.0% by weight ofsilica particles ( ®Sylysia 320 from Fuji, Japan) with d₅₀ of 2.5 μm andSPAN 98 of 1.4. The production conditions in each step of the processwere: Extrusion: Temperatures Layer A: 290° C. Layer B: 290° C. Layer C:290° C. Die width: 3.5 mm Take-off roll temperature: 30° C. LongitudinalStretching temperature: 80-126° C. stretching: Longitudinal stretchingratio: 4.5 Transverse Stretching temperature: 80-135° C. stretching:Transverse stretching ratio: 4.0 Setting: Temperature: 230° C. Duration:3 s

[0111] The film had the required high gloss and the required low haze.The film also has the desired winding performance and the desiredprocessing performance. The structure of the film and the propertiesachieved in films produced in this way are shown in Tables 2 and 3.

EXAMPLE 2

[0112] With Example 1 as a basis, the outer layer thickness for theglossy outer layers (A) and (C) was raised from 1.0 to 1.5 μm while theremainder of the film structure and the production method wereidentical. The result was an improvement in the winding performance ofthe film. The gloss of the film reduced marginally and the hazeincreased marginally.

EXAMPLE 3

[0113] Using Example 1 as a basis, the pigment concentrations in theouter layers (A) and (C) were now increased. The result was a marginalreduction in the gloss of the film and a marginal increase in the haze.In contrast, however, there was a further improvement in windability.

EXAMPLE 4

[0114] Using Example 3 as a basis, the outer layer thickness of theglossy outer layers (A) and (C) was raised from 1.0 to 1.5 μm while thefilm structure and the method of production were identical. The resultwas a further slight increase in the winding performance of the film,but the gloss reduced substantially and the haze increasedsubstantially.

EXAMPLE 5

[0115] Using Example 2 as a basis, the grain diameter in the outerlayers (A) and (C) was now increased at identical SPAN values. Theresult was a marginal reduction in the gloss of the film and a marginalincrease in the haze. In contrast, there was a further improvement inwindability.

EXAMPLE 6

[0116] Using Example 2 as a basis, another pigment system was added tothe glossy outer layers (A) and (C). Other than the abovementionedpigments, the two outer layers now also comprise 500 ppm of® Aerosil TT6000 (from Degussa), this being a fumed silica.

EXAMPLE 7

[0117] Using Example 6 as a basis, the coarser pigment Sylysia 340 wasnow used in the glossy outer layers (A) and (C) instead of Sylysia 320.

COMPARATIVE EXAMPLE 1

[0118] Using Example 1 as a basis, the outer layers (A) and (C) were nowformulated with a pigment system of the prior art as in U.S. Pat. No.3,154,461. The windability of the film deteriorated markedly, and theoptical properties also deteriorated. TABLE 2 Film Layer Median pigmentPigment thick- thicknesses diameter in layers concentrations ness Filmμm Pigments in layers μm ppm Example μm structure A B C A B C A B C A BC E 1 12 ABC 1.0 10 1.0 Sylysia 320 none Sylysia 320 2.5 2.5 1000 0 1000E 2 12 ABC 1.5 9 1.5 Sylysia 320 none Sylysia 320 2.5 2.5 1000 0 1000 E3 12 ABC 10 10 1.0 Sylysia 320 none Sylysia 320 2.5 2.5 1800 0 1800 E 412 ABC 1.5 9 1.5 Sylysla 320 none Sylysia 320 2.5 2.5 1800 0 1800 E 5 12ABC 1.5 9 1.5 Sylysia 430 none Sylysia 430 3.4 3.4 1000 0 1000 E 6 12ABC 1.5 9 1.5 Sylysia 320 none Sylysia 320 2.5 2.5 1000 0 1000 AerosilTT Aerosil TT 0.05 0.05 500 500 600 600 E 7 12 ABC 1.5 9 1.5 Sylysia 430none Sylysia 430 3.4 3.4 1000 0 1000 Aerosil TT Aerosil TT 0.05 0.05 500500 600 600 CE 1 12 ABC 1.5 9 1.5 d₅₀ = 5.5 none d₅₀ = 5.5 5.0 2.5 10000 1000 SPAN SPAN 98 = 2.2 0.04 98 = 2.2

[0119] TABLE 3 Section COF Average Values measured Side A roughnessR_(a) for gas flow Gloss Gloss Winding performance Processing againstSide Side Side Side Side and handling performance Example Side C A SideC A C Δp A C Haze Side A Side C E 1 0.46 55 55 85 85 0.167 200 2001.6 + + E 2 0.45 60 60 82 82 0.167 198 197 1.65 +(+) +(+) E 3 0.43 63 6279 78 0.167 192 192 1.8 ++(+) ++(+) E 4 0.43 64 63 78 76 0.167 190 1891.8 +++ +++ E 5 0.42 85 83 25 26 0.167 194 194 1.5 +++ +++ E 6 0.40 6262 80 80 0.167 192 192 1.8 ++(+) ++(+) E 7 0.41 85 84 25 26 0.167 191193 1.7 ++++ ++++ CE 1 0.48 55 50 90 90 0.167 190 190 1.75 − −

1. A multilayer, transparent, biaxially oriented polyester film madefrom at least one base layer (B) which comprises at least 80% by weightof thermoplastic polyester, and from, applied to this base layer (B), atleast one transparent, high-gloss outer layer (A), wherein thetransparent outer layer (A) also comprises an amount in the range from0.05 to 0.5% by weight, based on the total weight of the outer layer(A), of a pigment system which has the following features: a) the mediangrain diameter (d₅₀) is in the range from 1.5 to 5 μm and b) the spreadof the distribution of the grain size, expressed by the SPAN 98, is lessthan or equal to 1.9.
 2. The multilayer, transparent polyester film asclaimed in claim 1, wherein the transparent outer layer (A) comprises apigment system which has a median grain diameter (d₅₀) in the range from1.6 to 4.9 μm.
 3. The multilayer, transparent polyester film as claimedin claim 1, wherein the transparent outer layer (A) comprises a pigmentsystem which has a SPAN 98 of less than or equal to 1.8.
 4. The multilayer, transparent polyester film as claimed in claim 1, which has athree-layer structure with a base layer (B) and, arranged on the twosides of the base layer (B), outer layers (A) and (C), and which has anoverall thickness in the range from 3 to 80 μm, and wherein thethickness of the outer layers (A) and (C) is in the range from 0.1 to 5μm, and wherein outer layers (A) and (C) are of identical or differentthickness.
 5. The multilayer, transparent polyester film as claimed inclaim 1, whose gloss is greater than or equal to 170 and whose haze isless than or equal to 2.5%.
 6. The multilayer, transparent polyesterfilmas claimed in claim 1, whose roughness, expressed as its R_(a) value, isin the range from 30 to 150 nm, and whose value measured for surface gasflow is in the range from 4 to 200 s.
 7. The multilayer, transparentpolyester film as claimed in claim 1 whose planar orientation Δp isgreater than or equal to 0.165.
 8. A process for producing a multilayer,transparent polyester film as claimed in claim 1 by coextrusion, byfirst compressing, plasticizing, and thereby homogenizing the polyestersof the respective layers in extruders, at which juncture any additivesused may already be present in the respective polymer, and then bypressing the melts through a flat-film coextrusion die, and drawing offthe extruded multilayer film on one or more take-off rolls andsolidifying the same to give a prefilm, and then biaxially stretchingthe solidified prefilm, and heat-setting the biaxially stretched filmand, where appropriate, corona- or flame-treating the same on thatsurface intended for treatment, which comprises using a longitudinalstretching temperature in the range from 80 to 130° C. and using atransverse stretching temperature in the range from 90 to 150° C., andwhich comprises using a longitudinal stretching ratio in the range from2.5:1 to 6:1, and using a transverse stretching ratio in the range from3.0:1 to 5.0:1.
 9. The process as claimed in claim 8, wherein, afterstretching, the film is heat-set for a period in the range from 0.1 to10 s at a temperature of from 150 to 250° C.
 10. The process as claimedin claim 8, wherein one or both surfaces of the film is/are also corona-or flame-treated, the intensity of treatment set being such as to givethe film a surface tension in the range greater than or equal to 45mN/m.
 11. The process as claimed in claim 8, wherein cut materialarising during film production is reintroduced as regrind to theextrusion process in amounts in the range from 20 to 60% by weight,based on the total weight of the film.